In semiconductor fabrication processes, an electron beam (e-beam) emitted by a scanning electron microscope (SEM) is often used to measure dimensions of fine patterns. However, after a long time of use or due to environmental effects and/or wearing of SEM components, the quality of electron beam from the SEM may be changed. The quality of electron beam often refers to the quality of: cross-sectional shape and radius, resolution in all directions, etc. of the electron beam, which directly affect measuring accuracy. Therefore, the quality of the electron beam often needs to be examined before use of the SEM to measure fine patterns. Such examination ensures sufficient quality of the electron beam for measuring dimensions of the fine patterns.
Conventionally, prior to measuring dimensions of fine patterns, quality of electron beam can be examined as shown in FIGS. 1-3. Referring to FIG. 1, a first reference sample 10 and a second reference sample 20 are provided. The first reference sample 10 and the second reference sample 20 are usually provided by the manufacturer. The first reference sample 10 includes a first line pattern 11 in a Y-direction. The second reference sample 20 includes a second line pattern 21 in an X-direction, perpendicular to the Y-direction.
FIG. 2 depicts a first calibration pattern 11a and a second calibration pattern 21a. The first calibration pattern 11a is in a first coordinate system. The x-axis of the first coordinate system is X1 and the y-axis is Y1. The first calibration pattern 11a is a contour of the first line pattern 11 (shown in FIG. 1) scanned by the SEM prior to leaving the factory for end use. The second calibration pattern 21a is in a second coordinate system. The x-axis of the second coordinate system is X2 and the y-axis is Y2. The second calibration pattern 21a is a contour of the second line pattern 21 shown in FIG. 1) scanned by the SEM prior to leaving the factory for end use.
Referring to FIG. 3, the first line pattern 11 on the first reference sample 10 and the second line pattern 21 on the second reference sample 20 are scanned by a current electron beam of the SEM to obtain the contour 11a′ of the first line pattern 11 and the contour 21a′ of the second line pattern 21.
Still referring to FIG. 3, the contour 11a′ of the first line pattern 11 is compared with the first calibration pattern 11a in FIG. 2, and the contour 21a′ of the second line pattern 21 is compared with the second calibration pattern 21a in FIG. 2. When the contour 11a′ of the first line pattern 11 and the contour 21a′ of the second line pattern 21 are respectively close to or the same as the first calibration pattern 11a and the second calibration pattern 21a, the quality of the electron beam is considered high and can be used for subsequently measuring the dimensions of fine patterns.
However, as process nodes shrink, it is desired to increase measuring accuracy of fine patterns of semiconductor devices.